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Urinary System
Chapter 17
Bio 160
Introduction
• The urinary system consists of two kidneys
that filter the blood, two ureters, a urinary
bladder, and a urethra to convey waste
substances to the outside
Kidney
• The kidney is a reddish brown, bean-shaped
organ 12 centimeters long; it is enclosed in a
tough, fibrous capsule.
• The kidneys are positioned retroperitoneally
on either side of the vertebral column between
the twelfth thoracic and third lumbar vertebrae,
with the left kidney slightly higher than the
right.
Kidney
• Kidney Structure
– A medial depression in the kidney leads to a
hollow renal sinus into which blood vessels,
nerves, lymphatic vessels, and the ureter
enter.
– Inside the renal sinus lies a renal pelvis that
is subdivided into major and minor calyces;
small renal papillae project into each minor
calyx.
Kidney
– Two distinct regions are found within the
kidney: a renal medulla and a renal cortex.
• The renal medulla houses tubes leading to the
papillae.
• The renal cortex contains the nephrons, the
functional units of the kidney.
Kidney
• Kidney Functions
– The kidneys function to regulate the
volume, composition, and pH of body fluids
and remove metabolic wastes from the
blood in the process.
– The kidneys also help control the rate of red
blood cell formation by secreting
erythropoietin, and regulate blood pressure
by secreting renin.
Kidney
• Renal Blood Vessels
– The abdominal aorta gives rise to renal
arteries leading to the kidneys.
– As renal arteries pass into the kidneys, they
branch into successively smaller arteries:
interlobar arteries, arcuate arteries,
interlobular arteries, and afferent arterioles
leading to the nephrons.
Kidney
– Venous blood is returned through a series of
vessels that generally correspond to the
arterial pathways.
Kidney
• Nephrons
– Nephron Structure
• A kidney contains one million nephrons, each of
which consists of a renal corpuscle and a renal
tubule.
• The renal corpuscle is the filtering portion of
the nephron; it is made up of a ball of capillaries
called the glomerulus and a glomerular capsule
that receives the filtrate.
Kidney
• The renal tubule leads away from the
glomerular capsule and first becomes a highly
coiled proximal convoluted tubule, then leads to
the nephron loop, and finally to the distal
convoluted tubule.
• Several distal convoluted tubules join to become
a collecting duct.
Kidney
– Blood Supply of a Nephron
• The glomerulus receives blood from a fairly
large afferent arteriole and passes it to a smaller
efferent arteriole.
• The efferent arteriole gives rise to the
peritubular capillary system, which surrounds
the renal tubule.
Kidney
– Juxtaglomerular Apparatus
• At the point of contact between the afferent and
efferent arterioles and the distal convoluted
tubule, the epithelial cells of the distal tubule
form the macula densa.
Kidney
• Near the macula densa on the afferent arteriole
are smooth muscle cells called juxtaglomerular
cells.
• The macula densa together with the
juxtaglomerular cells make up the
juxtaglomerular apparatus.
Urine Formation
• Urine formation involves glomerular filtration,
tubular reabsorption, and tubular secretion.
• Glomerular Filtration
– Urine formation begins when the fluid
portion of the blood is filtered by the
glomerulus and enters the glomerular
capsule as glomerular filtrate.
Urine Formation
• Filtration Pressure
– The main force responsible for moving
substances by filtration through the
glomerular capillary wall is the hydrostatic
pressure of the blood inside.
– Due to plasma proteins, osmotic pressure of
the blood resists filtration, as does
hydrostatic pressure inside the glomerular
capsule.
Urine Formation
• Filtration Rate
– The factors that affect the filtration rate are
filtration pressure, glomerular plasma
osmotic pressure, and hydrostatic pressure
in the glomerular capsule.
– When the afferent arteriole constricts in
response to sympathetic stimulation,
filtration pressure, and thus filtration rate,
declines.
Urine Formation
– When the efferent arteriole constricts,
filtration pressure increases, increasing the
rate of filtration.
– When osmotic pressure of the glomerular
plasma is high, filtration rate decreases.
Urine Formation
– When hydrostatic pressure inside the
glomerular capsule is high, filtration rate
declines.
– On average, filtration rate is 125 milliliters
per minute or 180 liters in 24 hours, most of
which is reabsorbed farther down the
nephron.
Urine Formation
• Regulation of Filtration Rate
– Glomerular filtration rate is relatively
constant, although sympathetic impulses
may decrease the rate of filtration.
– Another control over filtration rate is the
renin-angiotensin system, which regulates
sodium excretion.
Urine Formation
• When the sodium chloride concentration in the
tubular fluid decreases, the macula densa senses
these changes and causes the juxtaglomerular
cells to secrete renin.
• Secretion of renin triggers a series of reactions
leading to the production of angiotensin II,
which acts as a vasoconstrictor; this may, in
turn, affect filtration rate.
Urine Formation
• Presence of angiotensin II also increases the
secretion of aldosterone, which stimulates
reabsorption of sodium.
• The heart can also increase filtration rate when
blood volume is high.
Urine Formation
• Tubular Reabsorption
– Changes in the fluid composition from the
time glomerular filtrate is formed to when
urine arrives at the collecting duct are
largely the result of tubular reabsorption of
selected substances.
– Most of the reabsorption occurs in the
proximal convoluted tubule, where cells
possess microvilli with carrier proteins.
Urine Formation
– Carrier proteins have a limited transport
capacity, so excessive amounts of a
substance will be excreted into the urine.
– Glucose and amino acids are reabsorbed by
active transport, water by osmosis, and
proteins by pinocytosis.
Urine Formation
• Sodium and Water Reabsorption
– Sodium ions are reabsorbed by active
transport, and negatively charged ions
follow passively.
– As sodium is reabsorbed, water follows by
osmosis.
Urine Formation
• Regulation of Urine Concentration and Volume
– Most of the sodium ions are reabsorbed
before the urine is excreted under the
direction of the hormone, aldosterone
– Normally the distal convoluted tubule and
collecting duct are impermeable to water
unless the hormone ADH is present.
Urine Formation
• Urea and Uric Acid Excretion
– Urea is a by-product of amino acid
metabolism; uric acid is a by-product of
nucleic acid metabolism.
– Urea is passively reabsorbed by diffusion
but about 50% of urea is excreted in the
urine.
– Most uric acid is reabsorbed by active
transport and a small amount is secreted into
the renal tubule.
Urine Formation
• Tubular Secretion
– Tubular secretion transports certain
substances, including penicillin, histamine,
phenobarbital, hydrogen ions and potassium
ions, from the plasma into the renal tubule.
– Active transport mechanisms move excess
hydrogen ions into the renal tubule along
with various organic compounds.
Urine Formation
– Potassium ions are secreted both actively
and passively into the distal convoluted
tubule and the collecting duct.
Urine Formation
• Study Analogy
– Pretend you are cleaning your garage but the big
door is stuck. You can only move things through
the smaller “people” door. So the cars and riding
lawn mower have to stay in the garage. This is
analogous to the pores in the glomerulus. They
are larger than ordinary capillary pores but still
not large enough to let everything out. So large
things like proteins stay in the blood. You have
decided to haul almost everything out that you can
fit through the smaller door. Out goes the hoses,
garden implements, lawn chemicals, recycling,
etc., without any sorting.
Urine Formation
You do this until you run out of time.
(Filtration = what fits goes through the pores
and is controlled by size and the pressures.)
After a short rest, you realize that you need
some of this stuff. So you exert some more
energy (active transport) and put some of
the materials back into the garage. For
example, 13 of the 27 hoses are still good so
they go back (like tubular reabsorption).
Urine Formation
The others are put out for the trash pickup
(analogous to going to the bladder). After sorting,
returning and discarding, you take one last look at
what is now in the garage. Do you really need 13
hoses? Isn’t that one a little holey? So you take it
back out of the garage and put it in the trash pile
with the others (just like tubular secretion, a last
chance to excrete something we don’t need). And
Viola! The garage (and your blood) is clean!
Urine Formation
• Urine Composition
– Urine composition varies from time to time
and reflects the amounts of water and
solutes that the kidneys eliminate to
maintain homeostasis.
– Urine is 95% water, and also contains urea,
uric acid, a trace of amino acids, and
electrolytes.
Urine Elimination
• After forming in the nephrons, urine passes
from the collecting ducts to the renal papillae,
then to the minor and major calyces, and out
the renal pelvis to the ureters, urinary bladder,
and finally to the urethra, which conveys urine
to the outside.
Urine Elimination
• Ureters
– The ureters are muscular tubes extending
from the kidneys to the base of the urinary
bladder.
– The wall of the ureter is composed of three
layers: mucous coat, muscular coat, and
outer fibrous coat.
Urine Elimination
– Muscular peristaltic waves convey urine to
the urinary bladder where it passes through
a flaplike valve in the mucous membrane of
the urinary bladder.
Urine Elimination
• Urinary Bladder
– The urinary bladder is a hollow, distensible,
muscular organ lying in the pelvic cavity.
– The internal floor of the bladder includes the
trigone, which is composed of the openings
of the two ureters and the urethra.
Urine Elimination
– The wall of the urinary bladder is made up
of four coats: inner mucous coat, submucous
coat, muscular coat made up of detrusor
muscle, and outer serous coat.
• The portion of the detrusor muscle that
surrounds the neck of the bladder forms an
internal sphincter muscle.
Urine Elimination
• Micturition
– Urine leaves the bladder by the micturition
reflex.
– The detrusor muscle contracts and the
external urethral sphincter (in the urogenital
diaphragm) must also relax.
Urine Elimination
– Stretching of the urinary bladder triggers the
micturition reflex center located in the sacral
portion of the spinal cord.
– Return parasympathetic impulses cause the
detrusor muscle to contract in waves, and an
urge to urinate is sensed.
Urine Elimination
– When these contractions become strong
enough, the internal urethral sphincter is
forced open.
– The external urethral sphincter is composed
of skeletal muscle and is under conscious
control.
Urine Elimination
• Urethra
– The urethra is a tube that conveys urine
from the urinary bladder to the outside.
– It is a muscular tube with urethral glands
that secrete mucus into the urethral canal.